Gecko is the html rendering engine used in Mozilla-based browsers. AIUI, the wine project is working to incorporate it as a replacement for the Windows rendering engine that's used by IE and various other programs like the help system.
Debian has an apt package for wine-gecko, and probably other distros do as well. Starting with the most recent version of wine (1.1.33), gecko will be automatically installed when a new wineprefix is created.
For more info directly from the wine project, see here:
http://wiki.winehq.org/Gecko